The use of acoustic (e.g., audible and/or ultrasonic) measurement systems in prior art downhole applications, such as logging while drilling (LWD), measurement while drilling (MWD), and wireline logging applications, is well known. Such acoustic measurement systems are utilized in a variety of downhole applications including, for example, borehole caliper measurements, measurement of drilling fluid properties, and the determination of various physical properties of a formation. In one application, acoustic waveforms may be generated at one or more transmitters deployed in the borehole. The acoustic responses may then be received at an array of longitudinally spaced apart receivers deployed in the borehole. Acoustic logging in this manner provides an important set of borehole data and is commonly used in both LWD and wireline applications to determine compressional and shear wave velocities (also referred to as slowness) of a formation.
It will be appreciated that the terms slowness and velocity are often used interchangeably in the art. They will likewise be used interchangeably herein with the understanding that they are inversely related to one another and that the measurement of either may be converted to the other by simple and known mathematical calculations. Additionally, as used in the art, there is not always a clear distinction between the terms LWD and MWD. Generally speaking MWD typically refers to measurements taken for the purpose of drilling the well (e.g., navigation) whereas LWD typically refers to measurements taken for the purpose of analysis of the formation and surrounding borehole conditions. Nevertheless, these terms are herein used synonymously and interchangeably.
In the analysis of acoustic logging measurements, the received acoustic waveforms are typically coherence processed to obtain a time-slowness plot. In a time-slowness plot, also referred to as a slowness-time-coherence (STC) plot or a semblance plot, a set of several signals from the array of acoustic receivers is processed with the incorporation of separate time shifts for each received signal. The separate time shifts are based on a slowness value assumed for the purpose of processing the waveforms. The processing provides a result, known as coherence, which can signify the presence of a discernable signal received by the separate receivers. In this manner compressional and shear wave arrivals can be discerned in the received waveforms, leading to determinations of their velocities. The determined compressional and shear wave velocities are related to compressive and shear strengths of the surrounding formation, and thus provide useful information about the formation.
Subterranean earth formations are rarely homogeneous, but instead include geologic features such as fractures, thin beddings, microlayering in shales, and strata of differing compositions, as well as oil and gas deposits. Geologic features in formations generally lead to stresses and formation density variations, which change the way acoustic waves propagate in the formation. The distribution of fractures and other geologic features in the vicinity of a borehole can cause acoustic velocity anisotropy. The measured compressional and shear velocities may then depend on the toolface angle, an azimuthal direction about the borehole defined with respect to a reference direction.
In acoustically slow formations, in which the velocity of formation shear waves is less than the speed of sound in the drilling fluid (mud), shear wave slowness determination is known to be complicated by poor transmission of shear wave energy across the boundary between the formation and the borehole. Techniques to address this difficulty by estimating shear wave slowness from the guided wave slowness of, for example, excited pure modes also present problems. These problems include dispersive effects in borehole guided wave propagation, difficulty in exciting pure modes, and the expense and complexity of transducer arrangements and timing in LWD tools tailored to produce a pure mode and to suppress other modes.
The difficulty in generating such borehole guided waves is also known to be further exacerbated by tool eccentricity in the borehole (e.g., in highly deviated wells in which the tool typically lies on the low side of the borehole). Effective generation and detection of borehole guided waves can depend on transducer standoff. It can be difficult to obtain shear wave slowness measurements for particular toolface angles when the LWD tool is eccentered, particularly in a large diameter borehole or in a portion of a borehole having a larger diameter. Such sensitivity to tool eccentricity can hinder shear anisotropy determination.
Therefore, there exists a need for an improved method for acoustic anisotropy determination during logging while drilling. In particular, there is a need for an improved method for acoustic anisotropy determination (including shear wave anisotropy) that is less sensitive to tool eccentricity and is also effective in acoustically slow formations during logging while drilling operations.